When pastels get too small, or there is good dust left after a painting, I carefully keep it to recycle into new sticks. I like using the reconstituted ones very much, since they are such a mixture of in-between colors.
We have a dedicated mortar and pestle to do the job. All else that is needed is a bit of distilled water to reconstitute them. The pigment already has binder in them. I mix very soft and harder pastels all together, taking the mix of color and its accompanying hardness as it comes. I used to try to make them into sticks with a palette knife, but I’ve found I like using palm rolled ovals (slightly flattened) better.
In the photos here you can see the process. I just love looking at the colors as they are ground together!